This invention relates to a high frequency oscillator utilizing a phase synchronizing or locking loop and is useful as a local oscillator of a FM receiver, television receiver, communicating system or other superheterodyne system.
In the prior art local oscillator utilizing a variable capacitor, for example, the oscillation frequency varies with temperature or moisture variation or with the elapse of time. Although a receiver provided with an automatic frequency control circuit (AFC) has been proposed to obviate this difficulty, there is a difficulty that the reception of the signals from broadcasting stations with small antenna input level are interfered with the signals from neighboring broadcasting stations with large antenna input level. For this reason, as an oscillator having a sufficient frequency stability a frequency synthesizing type oscillator of the phase locking type and utilizing a programmable frequency divider has recently been used. FIG. 1 shows a block diagram of one example of such an oscillator in which the oscillation frequency of a voltage controlled oscillator 1 is divided by a programmable frequency divider 2 and the phase of the output of the frequency divider 2 is compared by means of a phase detector 3 with the phase of the oscillation frequency of a reference oscillator 7 operating at substantially the same frequency. The detector output corresponding to the phase difference is filtered by a low pass filter 4. The output of the filter 4 is fed back to the oscillator 1 for stabilizing the oscillation frequency thereof. When such an oscillator is used as a local oscillator of a receiver it is possible to vary the receiving frequency by varying the ratio of the frequency division of the programmable frequency divider 2 by manipulating a frequency division ratio setting circuit 6 whereby it is possible to obtain a receiver of excellent characteristic. However, such an oscillator cannot be used extensively because the programmable frequency divider and the frequency division ratio setting circuit are extremely complicated and expensive.